Wiring materials such as insulated wires and heat-shrinkable tubes for use in harnesses to be attached in automotive engine rooms should employ or comprise resin materials excellent not only in flexibility but in flame retardancy, heat resistance, thermal aging resistance, oil resistance, and abrasion resistance, from the standpoints of harness handleability, etc. Hitherto, several kinds of polymers including crosslinked poly(vinyl chloride), crosslinked polyethylene, and fluorinated polymers have been properly used according to required temperature ratings.
In the field of motor vehicles, on the other hand, there is a trend toward weight reduction in automotive parts for improving fuel efficiency so as to cope with environmental problems. With respect to insulated wires also, investigations have been made on the use of thinner conductors and the thickness reduction in insulating coating layers.
The insulated wires currently most frequently used in engine room harnesses are the so-called AVX (automotive low-voltage wire insulated by crosslinked PVC; temperature rating, 110.degree. C.) and AEX (automotive low-voltage wire insulated by crosslinked PE; temperature rating, 120.degree. C.) each having an insulator thickness of 0.5 mm. However, from the standpoint of the desire for thickness reduction, thinly coated wires have been put to practical use, such as the so-called AVSSX (automotive low-voltage wire insulated by ultrathin crosslinked PVC; temperature rating, 110.degree. C.) and AESSX (automotive low-voltage wire insulated by ultrathin crosslinked PE; temperature rating, 120.degree. C.) each designed to have an insulator thickness of 0.30 mm.
However, the desire for thickness reduction becomes stronger year by year, and investigations are being made in order to develop an insulated wire in which the thickness of the insulating layer has been reduced to 0.20 mm or 0.10 mm.
Furthermore, with the trend toward increase in automotive performance, the engine room is coming to heat up to a higher degree and the number of electrical equipment parts is increasing. The requirements for improvements in the thermal aging resistance of the harnesses to be connected to these electrical equipment parts also are becoming severer year by year. As a result, there is a desire for an insulated wire having an insulator thickness of 0.2 mm and a temperature rating of 125.degree. C. or 150.degree. C.
The properties required of automotive electric wires are prescribed in detail in standards including ISO 6722. Among these properties, abrasion resistance and thermal aging resistance are properties which are thought to be more difficult to attain as the insulator becomes thinner.
Hitherto, abrasion resistance has been evaluated by the so-called tape abrasion test method illustrated in FIG. 1. However, with decreasing insulator thickness, the scrape abrasion test method illustrated in FIG. 2 has come to be used for the evaluation because it has become necessary to more properly evaluate reliability concerning abrasion resistance.
FIG. 1 is a diagrammatic view illustrating the tape abrasion test method conventionally used for examining the abrasion resistance of electric wires.
In this test, a load 3 of 453 g is imposed on an electric wire sample 1. A #150 sandpaper 2 is placed beneath the sample 1, and is caused to run at a constant rate to measure the distance through which the sandpaper has run until the conductor in the insulated wire 1 is exposed.
FIG. 2 is a diagrammatic view illustrating the scrape abrasion test method used in the present invention for examining the abrasion resistance of electric wires.
In this scrape abrasion test method, a load 3 of 714 g is imposed on a steel bar 4 having an outer diameter of 0.45 mm. This steel bar 4 is reciprocated on an electric wire sample 1 to scrape the sample, and the number of reciprocations required for the steel bar 4 to come into electrical contact with the conductor of the insulated wire is determined.
The durability level generally required in the above test is 300 reciprocations or higher. However, the thinner the insulator layer, the more the desired durability is difficult to attain.
With respect to thermal aging resistance, on the other hand, an insulated wire is required to have such a property that after the sample is subjected to 10,000-hour thermal aging at a rated temperature, the electrical properties of the sample and the mechanical properties of the insulator are higher than given levels. This requirement also tends to become more difficult to satisfy as the insulator layer becomes thinner.
If the electric wires insulated by crosslinked poly(vinyl chloride) or crosslinked polyethylene are designed to have an insulator thickness reduced to below 0.30 mm, it is difficult to attain the abrasion resistance of 300 reciprocations or higher and to meet the thermal aging resistance of 120.degree. C. in terms of temperature rating.
In contrast, electric wires insulated by a fluorinated polymer, even when having a reduced insulator thickness, can satisfy the requirements concerning abrasion resistance and thermal aging resistance. However, these insulated wires have a drawback that the parts to which they are applicable are limited mainly because of their cost. It has hence become necessary to investigate new insulating materials.
Use of various thermoplastic elastomers is being investigated as insulating materials which may satisfy such requirements concerning thickness reduction and flexibility, abrasion resistance, thermal aging resistance, cost, etc.
Among these elastomers, polyester type thermoplastic elastomers (hereinafter abbreviated as polyester elastomers) are the most attractive polymers because they are excellent not only in flexibility but in abrasion resistance and thermal aging resistance.
The polyester elastomers are block copolymers comprising a crystalline hard segment such as poly(butylene terephthalate), made up of repeating units derived from terephthalic acid and 1,4-butanediol, and a noncrystalline soft segment derived from a polyether glycol, e.g., polytetramethylene glycol, or .epsilon.-caprolactone.
Of these polymers, a block copolymer elastomer comprising poly(butylene terephthalate) as a hard segment and an aliphatic polyester derived from .epsilon.-caprolactone as a soft segment is known as a polymer having excellent thermal aging resistance.
This block copolymer elastomer is produced, for example, by a process comprising polymerizing terephthalic acid with 1,4-butanediol using a polymerization catalyst, e.g., an organotitanium catalyst, to obtain a prepolymer and adding .epsilon.-caprolactone to the prepolymer to further conduct polymerization. By changing the proportion of the hard segment to the soft segment, various grades have been developed which range in modulus of elasticity from 1,000 to 10,000 kg/cm.sup.2.
The present inventors used two polyester elastomers having moduli of elasticity of about 1,500 kg/cm.sup.2 and about 5,500 kg/cm.sup.2, respectively, to coat over a conductor having an outer diameter of 0.80 mm by extrusion coating with an extruder in two thicknesses of 0.20 mm and 0.50 mm for each elastomer. The insulated wires thus obtained were examined for abrasion resistance and thermal aging resistance.
Abrasion resistance was evaluated by the scrape abrasion test method illustrated in FIG. 2. With respect to thermal aging resistance, 0.20 mm-thick insulator samples (length, 200 mm) were subjected to a thermal aging test by the Arrhenius method, in which the samples were hung down in three Geer ovens respectively controled so as to have temperatures of 140.degree. C., 160.degree. C., and 180.degree. C. and the time period required for each insulator sample to have an elongation reduced to 50% was measured. From these results, the temperature at which 10,000-hour aging resulted in an elongation of 50%, i.e., the temperature rating for 10,000-hour aging, was determined.
The results obtained are shown in Table 1.
TABLE 1 ______________________________________ Modulus of elasticity of 1500 5500 polyester (kg/cm.sup.2) Melting point of 200 216 polyester (.degree. C.) Thickness of insulator 0.2 0.5 0.2 0.5 (mm) Abrasion resistance 15 180 130 1700 (number of reciprocations) Thermal 200.degree. C. 240 80 aging life (hour) 180.degree. C. 670 288 160.degree. C. 1900 530 Temperature rating for 131 102 10,000-hr aging (.degree. C.) Volume resistivity (.OMEGA.cm) 1.3E + 12 4.4E + 13 ______________________________________
As shown in Table 1, the electric wires insulated by the polyester elastomer having a modulus of elasticity of 1,500 kg/cm.sup.2 had an insulator temperature rating of about 131.degree. C., showing that these insulated wires had thermal aging resistance sufficient for 125.degree. C. rating. However, with respect to abrasion resistance, the insulated wires having insulator thicknesses of 0.20 mm and 0.50 mm, respectively, had numbers of reciprocations of 15 and 180, respectively, which were below the required value of at least 300. That polyester was thus found to have an insufficient abrasion resistance for use as an insulator for thinly insulated wires.
On the other hand, the electric wires insulated by the polyester elastomer having a modulus of elasticity of 5,500 kg/cm.sup.2 had an abrasion resistance as high as 1,700 reciprocations when the insulator thickness was 0.50 mm. However, the insulated wire having an insulator thickness of 0.20 mm had an abrasion resistance of 130 reciprocations, which was below the required value of at least 300. Furthermore, this insulator had a temperature rating as poor as about 102.degree. C. The above polyester was thus found to be insufficient in both abrasion resistance and thermal aging resistance when used as the insulator of thinly insulated wires.
In addition, the following was found. The insulator made of the polyester elastomer having a modulus of elasticity of 5,500 kg/cm.sup.2 had relatively satisfactory insulating properties with a volume resistivity of 4.4.times.10.sup.13 .OMEGA.cm. However, the insulator made of the polyester elastomer having a modulus of elasticity of 1,500 kg/cm.sup.2 had a volume resistivity as low as 1.3.times.10.sup.12 .OMEGA.cm, showing that this elastomer was somewhat problematic in electrical insulating properties.
From the standpoint of improving the thermal aging resistance of polyester resins, JP-A-9-227661 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a polyester resin composition which is crosslinkable by irradiation with actinic energy rays, e.g., electron beams.
The polyester resin disclosed in the above reference comprises (1) units derived from an acid ingredient (A) comprising (A1) terephthalic acid or a lower alkyl ester thereof, (A2) an aromatic dicarboxylic acid other than terephthalic acid or a lower alkyl ester thereof, and (A3) an aliphatic dicarboxylic acid and/or an aliphatic hydroxycarboxylic acid and (2) units derived from a glycol ingredient (B) comprising (B1) an aliphatic linear diol having 2 to 4 carbon atoms and/or (B2) an aliphatic linear diol having 5 or more carbon atoms, wherein the molar ratio of (A1)/(A2)/(A3) is (35-75)/(20-30)/(20-50) and the molar ratio of (B1)/(B2) is (70-100)/(0-30).
Also disclosed in the above reference are: a random copolyester resin produced through polymerization for which the monomer ingredients (A) and (B) are introduced into a reactor at a time; and a resin composition containing a polyfunctional monomer having the effect of accelerating crosslinking.
The present inventors evaluated the above prior art technique in the following manner. An example of the above polyester resin was produced by introducing (A1) dimethyl terephthalate, (A2) dimethyl isophthalate, (A3) .epsilon.-caprolactone, and (B) 1,4-butanediol into a reactor at a time in a molar proportion of 4.4/1.9/3.7/10.0 and polymerizing the same. This polyester had a melting point of 140.degree. C. (melt flow rate, 39; modulus of elasticity, about 1,400 kg/cm.sup.2). A hundred parts by weight of the polyester was melt-mixed with 10 parts by weight of trimethylolpropane triacrylate as a polyfunctional monomer and 1 part by weight of a hindered phenol antioxidant (Irganox 1010, trade name, manufactured by Ciba-Geigy Ltd.) by means of a twin-screw extruder to prepare a resin composition. A conductor having an outer diameter of 0.80 mm was extrusion-coated with the composition in thicknesses of 0.5 mm and 0.20 mm. The coated conductors were irradiated with electron beams at an accelerating voltage of 1 MeV in a dose of 200 kGy. The insulated wires thus obtained were evaluated for abrasion resistance and thermal aging resistance.
TABLE 2 ______________________________________ Modulus of elasticity of polyester 1400 (kg/cm.sup.2) Melting point of polyester (.degree. C.) 140 Thickness of insulator (mm) 0.2 0.5 Abrasion resistance (number of 13 370 reciprocations) Thermal aging 200.degree. C. melted melted life (hour) 180.degree. C. melted melted 160.degree. C. 3800 -- Temperature rating for 10,000-hr aging -- -- (.degree. C.) ______________________________________
As a result, as Table 2 shows, the insulated wire having an insulator thickness of 0.5 mm had an abrasion resistance of 370 reciprocations, whereas that having an insulator thickness of 0.20 mm had an abrasion resistance of 13 reciprocations. Namely, the abrasion resistance of each insulated wire was below the required value of at least 300.
With respect to thermal aging resistance, the samples suffered melting and sagging in the thermal aging test at 180.degree. C. and 160.degree. C. and were unable to retain their shape. Hence, elongation measurement was impossible. Samples having a thickness of 0.5 mm also were tested. As a result, the same melting phenomenon occurred and elongation measurement was impossible.
JP-A-55-56135 discloses a process for producing a molded polyester elastomer crosslinked by irradiation with actinic energy rays, e.g., .gamma.-rays, likewise from the standpoint of improving the thermal aging resistance and other properties of polyester resins.
This process comprises subjecting an acid ingredient (1) comprising an aromatic dicarboxylic acid (A) and an aliphatic dicarboxylic acid and/or an aliphatic hydroxycarboxylic acid (B) to polycondensation with a diol ingredient comprising an aliphatic glycol (C) to produce a linear copolyester, molding the copolyester, if desired after an aliphatic unsaturated compound is incorporated thereinto, and then irradiating the molding with a radiation to crosslink the copolyester.
In Examples given in the above reference, crosslinking accelerators such as diallylglycidyl isocyanurate and triallyl isocyanurate are used as the aliphatic unsaturated compound to obtain crosslinkable polyester resins.
However, the above molded polyester elastomer was found to have the same problems as the polyester resin disclosed in JP-A-9-227661.
Although there is a description in JP-A-55-56135 to the effect that various compounds may be copolymerized as the aliphatic unsaturated compound in the form of a dicarboxylic acid, no experiments are given therein in which such various compounds are actually used. There is no description in the above reference concerning specific conditions for producing such a copolyester resin, properties of the resin obtained, etc.
As described above, although various polyester resins excellent in flexibility, abrasion resistance, and thermal aging resistance have been developed, use of these prior art resins in thinly insulated wires encounters difficulties in attaining both abrasion resistance and thermal aging resistance or the like. There has hence been a desire for the development of a polymer which not only has excellent flexibility but satisfies requirements concerning abrasion resistance, thermal aging resistance, electrical insulating properties, etc.